[PC Bus Description(s)]
[ISA Card Form Factors] [PC Interface ICs]
[PC Connectors] [PC-XT / PC-AT Pinout] [PCAT Cards] [PC Chassis]
Two different IBM PC-AT cards, with board dimensions are shown below.
The lower card is a normal PC-AT card, note the skirt just in front of J2.
Both expansion card formats carry the PC-AT interface, labeled J1 & J2.
The PCXT bus uses the J1 A/B rows, and a PCAT bus [ISA Bus] uses the J1 [A/B rows] and J2 [C/D rows] connectors. The fingers are copper strips on the PWB spaced on 0.1 inch centers. The PCAT bus was an up-grade to the original PCXT bus. The Pin-Out for both bus types are shown in the table below. The connector positions and relationship to the board are shown in the two figures at the top of the page. Both figures show a PCAT card, but the first figure also shows an attached PCI bus. Example dimensions for the card sizes are provided. Some of the signal names provided in the pin-out table are not correct terms. For example the Address lines called out above would be called SA# [SA19 - SA0], and the Data lines would be called SD# [SD15 - SA0]. The IBM compatible AT card [ISA] used the standard (edge) connector provided by the XT bus and added an additional edge connector behind that with the same pin-spacing @ 0.1 inch center-to-center. The additional connector has only 36 fingers (18 per side), while the XT connector had 62 fingers (32 per side). The Personal Computer Motherboard could then accept either an 8 or 16 bit card in an 8 bit slot (XT), or (if the connector was provided) a 16 bit card in an AT slot. A maximum number of 8 Expansion slots were provided on IBM compatible Motherboards. The ISA clock ran at around 10MHz. The ISA bus is obsolete and should not be used with new computer systems.
|1||Channel Check||Ground||System Enable||Memory 16 bit select|
|2||Data 7||Reset||Unlatched Address 23||I/O 16bit Chip Select|
|3||Data 6||+5v||Unlatched Address 22||IRQ10|
|4||Data 5||IRQ9||Unlatched Address 21||IRQ11|
|5||Data 4||-5v||Unlatched Address 20||IRQ12|
|6||Data 3||DMA Request 2||Unlatched Address 19||IRQ15|
|7||Data 2||-12v||Unlatched Address 18||IRQ14|
|8||Data 1||Zero Wait State||Unlatched Address 17||DMA ACK0|
|9||Data 0||+12v||Memory Read||DMA Request 0|
|10||I/O Channel Ready||Ground||Memory Write||DMA ACK5|
|11||Address Enable||Real Memory Write||Data 8||DMA Request 5|
|12||Address 19||Real Memory Read||Data 9||DMA ACK6|
|13||Address 18||I/O Write||Data 10||DMA Request 6|
|14||Address 17||I/O Read||Data 11||DMA ACK7|
|15||Address 16||DMA ACK3||Data 12||DMA request 7|
|16||Address 15||DMA Request 3||Data 13||+5v|
|17||Address 14||DMA ACK1||Data 14||Master|
|18||Address 13||DMA Request 1||Data 15||Ground|
|26||Address 5||DMA ACK2|
|27||Address 4||Terminal Count|
|28||Address 3||Address Latch En|
The pins labeled as N/A [Not Applicable] are not used, and don't exist on the expansion board.
The signal assignments or pin-outs listed above are stable and will not be changing or modified.
The PC-AT Physical Interface is no longer being worked on by any standards organizations.
The Industry Standard Architecture Bus is an out-dated PC expansion bus.
Personal computers now use the PCIe interface as the back-bone for expansion cards in PCs.
Back to the main PC AT Bus page for a description, IC vendors and connector manufacturers.
ISA Board Vendors; PC-AT Board Manufacturers [PC-AT cards are still listed].
As already stated on the related PCAT pages and PCXT topics, the PC-AT expansion card is an out dated interface.
The boards mechanical dimensions, signal names and pin descriptions shown above are still accurate, but the standard is obsolete.
In fact because the PCAT specification is frozen, or no longer actively worked implies that the J2 pin outs will remain unused [shown as N/A]..
The card size will remain fixed at the maximum dimensions specified in the graphics above.
Topic Navigation: Engineering Home > Interface Buses > Personal Computer Buses > PC-AT Expansion Slot > PC-AT Expansion Board.